1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
Here, there will be explained a principle of the EUV light generation in the LPP type light source apparatus. Target material supplied into a vacuum chamber is irradiated with a laser beam, and the target material is excited into plasma state. From this plasma, light-components with various wavelengths including the EUV light are radiated. Then, a light component with a desired wavelength (e.g., component with a wavelength of 13.5 nm) is selectively reflected and collected by using an EUV collector mirror and outputted to exposure equipment. On the reflecting surface of the EUV collector mirror, for example, a multi-layered film (Mo/Si multi-layered film) is formed by alternately stacking a molybdenum (Mo) thin film and a silicon (Si) thin film.
In such an LPP type EUV light source apparatus, particularly in a case of using a solid target, there is a problem about influence of neutral particles or ions emitted from the plasma. Since the EUV collector mirror is disposed close to the plasma, the neutral particles emitted from the plasma attach to the reflecting surface of the EUV collector mirror to deteriorate reflectivity of the mirror. Meanwhile, the ions emitted from the plasma cut out the multi-layered film formed on the reflecting surface of the EUV collector mirror. Here, flying particles from the plasma including neutral particles and ions and remains of the target material are called as debris.
As a related technology, Japanese Patent Application Publication JP-P2006-244837A discloses a laser plasma radiated light generating apparatus comprising means for supplying material, which is solid at a room temperature, continuously for a long time by using a simple device operated with a simple adjustment. The laser plasma radiated light generating apparatus ejects a solution containing fine particles from a nozzle to generate a liquid jet or a liquid droplet, irradiates the liquid jet or liquid droplet with a pulse laser beam to evaporate the solvent thereof by heat, and consecutively after a delay time of 0.1 μs or more, irradiates the heated liquid jet or liquid droplet with another pulse laser beam to generate-plasma.
Further, Japanese Patent Application Publication JP-A-11-250842 discloses a laser plasma light source which generates little debris and has a high conversion efficiency using a solid target. The laser plasma light source uses a solid target formed with a hollow at a part thereof irradiated with a laser beam, ablates an inside wall of the hollow by using a pulse laser for ablation, irradiates the hollow with a pulse laser beam for heating, after having waited for generation of a high density portion of evaporated material in the space within the hollow, and then excites the high density portion into high temperature plasma to generate a radiation.